1. FIELD OF INVENTION
This invention relates to a method and apparatus for indicating the hottest spot temperature and controlling the temperature of multi-phase power transformers of the type used by public utilities. This type of transformer typically has its transformer windings immersed in a liquid coolant. During operation of the power transformer it is important to know the temperature of the hottest transformer winding, the maximum temperature it has reached, when cooling fans should be turned on and off and when power to the transformer should be turned off. This invention is a variation of co-pending application Ser. No. 772,133 filed Aug. 30, 1985 for a Modular Electronic Temperature Controller, by Joseph F. Foster.
2. DISCUSSION OF PRIOR ART
It is known from prior systems to simulate the winding temperature by measuring the top oil temperature of the transformer and biasing the thermometer reading by an amount proportional to transformer load, or winding current, so as to indicate winding temperature. Such a system employs a thermometer with a sensing element measuring top oil temperature, a current transformer to provide a signal proportional to transformer load, a heater coil to provide the bias and an auto transformer to adjust the current supplied to the heater coil; often this simulation is applied to each transformer winding. That is, a three phase transformer usually has three complete simulators one for each winding.
It is a object of this invention, rather than have biasing heaters to modulate the actual oil temperatures in a transformer, to provide a method and apparatus to electronically determine the winding temperature of the hottest winding based on the oil temperature in the transformer and the increased temperature that results from the largest current flowing through the transformer windings.
It is a further object to provide an improved adjustable transformer temperature controller which may be adjusted or programmed to provide a variable temperature increment to the oil temperature in accordance with the transformer manufacturers specification for temperature change at various transformer loads or winding currents.